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In the uplink transmission of Orthogonal Frequency Division Multiple Access (OFDMA) systems, the subcarriers are assigned to uplink users by the system scheduler. Since each user only applies a portion of the subcarriers and the users experience different multipath channels, conventional channel estimation approach using uniformly placed pilot tones and uniform power allocation is not applicable. In this paper, we assume the places of the pilot tones have been decided by the scheduler and consider optimal power allocation of the pilot symbols, so as to minimize the mean square error (MSE) of channel estimation. The problem is formulated as a classical semidefinite programming (SDP) problem, and hence it can be solved with polynomial complexity.